In many applications, it is important to detect and accurately measure thin layers. Oxide build-up in fossil boiler tubes, epoxy (carbolene) and other protective coatings (e.g., chrome) on metal surfaces are examples of this need. In fossil boiler tubes, oxide layers form and grow in thickness during normal operation. Measurements of oxide thickness can be used to assess operation and maintenance, or simply to monitor corrosion rates. Therefore, it is desirable to accurately measure the thickness of oxide which is present. The thickness information can then be used to support engineering calculations of heat transfer characteristics and estimates of the remaining life of the tubes.
Typically, the thickness measurement is performed using ultrasonics. In order to improve the resolution of ultrasonic measurement, it is often desired to deconvolve a resulting waveform to recover the "impulse" that was used to generate the response. The "impulse" is the spike or square wave pulse used to excite the transducer. It is a short duration event which, when convolved with the transfer function of the transducer and material, results in a long duration (ringing) response. This long duration response tends to limit the time resolution of separate events, such as a thin oxide layer adhering to a tube wall.
Other systems which have been utilized for oxide measurement typically requires higher frequencies, about 25 MhZ, to achieve a resolution of 6 to 10 mils of oxide thickness. This is a deficiency since more surface preparation is required for the higher frequencies. Also, the results can be ambiguous due to the long duration of the ultrasonic response. Although high frequencies do provide higher resolution, the instrumentation and transducers required by such frequencies are more expensive. Furthermore, it is difficult to obtain quality, low-noise signals.
Another technique to ultrasonically measure thin layers is to look at the waveforms several multiples later in time. This compensates for the lack of resolution by magnifying the time difference between two events. For instance, the first multiple will increase the time difference by a factor of two, the second multiple by a factor of three, and so forth. Unfortunately, attenuation and other effects degrade the signal-to-noise ratio.
An alternate approach is to use a rule-based signal processing scheme that extracts the separate events in the ultrasonic waveform. Various signal features are examined to indicate if and where two separate events are occurring. Unfortunately, the algorithms have proven to be unreliable. The unreliable performance is primarily due to the complex nature of the waveform.
Hence, there is a need to provide an ultrasonic measuring system which utilizes low frequency ultrasound to minimize the amount of surface preparation necessary, yet provides resolution comparable to systems using high frequencies. There is a further need to provide a measuring system which accurately measures thin films, economically and in a reasonable amount of time.